So uh, how is adding co-op to a survival horror game that focuses on themes of isolation a good thing?

If they did it right and introduced positive changes then it would be a good thing. Not every sequel has to follow the same formula for a series to be good. I'd enjoy more shaking it up in popular series instead of rehashing

Really? And why is that? Because I'm on chapter 12, and am enjoying it just fine. It's really no different than DS 1 and 2. It was starting to get a bit too familiar and then I made it to Tau Volantis. I'm liking this creepy, cold as hell planet. Lots of mystery, too. I especially like the new "make your own weapons" feature. I've spent far too much time customizing my weapons. The possibilities are crazy. The story is wonky so far, but not much different than the past two. The only real problem I have is trying to fight a very action game kind of boss, but with this clunky control system. Fortunately this is rare.

Does anyone know if you get bonus space credits or some shit for beating DS1 and 2?

Bonus space credits? I have no idea. I played DS 1 & 2 on the 360, but am playing DS3 on the PS3. It didn't occur to me that there would be any kind of reward, and I prefer gaming on my PS3, so I went that route. If there is anything, I haven't heard about it. None of the reviews I read said anything either, so it's doubtful.